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REMOTE CONTROL BY BOB YOUNG Internal combustion motors for R/C models; Pt.3 This month, we continue our discussion on the use of lubricating oil in fuel mixtures. Castor oil has the advantage of being good as an upper cylinder lubricant but its residue after the fuel is burnt is a real problem to clean off models. Here let me state, knowing that once again I am about to bring down the wrath of the gods, that there is still no substitute for castor oil. It is still one of the best upper cylinder (high temperature) lubricants available. It has been said that castor oil runs towards the heat, whereas all other oils run away; a trifle poetic perhaps but it does sum up the position rather nicely. There are other benefits of synthetic oils which are mainly to do with less drag on moving components, ageing Modern wisdom has it that synthetic oils and castor oil can be mixed and the fuel mixtures quoted range from 5% Gloglide and 95% methanol (9 .5:0.5) to 10% synthetic, 10% castor oil and 80% methanol (80:10:10). The key factor always, it seems, is to use as little castor oil as possible because of the undesirable side effects of this substance. Personally, I do not recommend oil mixtures as low as these and they are quoted as a guide to popular opinion only. "The basic problem in choosing a lubricant & ratio arises from the fact that every model calls for a different mix" of residues and a lower glazing factor, but to delete castor oil from the fuel completely, to my mind at least, leaves the motor vulnerable to seizing under some extreme conditions. It can be done but the modeller must know exactly what he is about. The two most common synthetic oils used, in Sydney at least, are Gloglide and Synlube. Klotz is another popular oil, particularly for 4stroke engines, as it does not gum up the valve gear as badly as castor oil. 88 SILICON CHIP The basic problem in choosing a lubricant and ratio arises from the fact that every model calls for a different mix. Take for example, a 60 2stroke Schnurle ported motor, one of the work horses of the model aircraft movement. In the good old days, before noise pollution laws prohibited behaviour such as I am about to describe, I used to run an O.S. 60 in a very sleek aerobatic aircraft with a very slender nose. This model was very fast which meant that the revs had to be high to maintain forward speed. This suited the Schnurle ported motor well, as they tend to prefer high rev (14,000 rpm plus), low torque applications. Thus, I found that a 10x7 prop was ideal for this model. The cylinder head was fully exposed to the slipstream for maximum cooling, while the muffler used was a low back pressure, flowthrough model which placed the minimum of thermal stress on the motor. (It also placed a minimum of emphasis on noise reduction). Thus· we have an application in which the piston has a minimum of dwell time at TDC (top dead centre), efficient exhaust gas extraction and a high volume of cooling air passing over the fins on the cylinder head and muffler. This application was one in which I could have used a minimum of oil had I wished. I never did and always stuck with 4:1 methanol/castor oil. I also used to run my motors towards the lean side for maximum revs, a potentially dangerous situation as we shall see, but I always used a fuel pump and a fuel tank on the centre of gravity in this particular model. I flew with the same motor for many years and never replaced the piston or cylinder. Cowled engine bay If we now take that same motor and place it inside a fully cowled engine bay on a scale model of the same weight, we will suddenly be confronted with a vastly different situation. Assuming that the model is of a radial engined aircraft, the prop will need to be larger to compensate for Protect _your valuable issues Silicon Chip Binders This shot from the Bob Young archives shows a Silvertone Weightlifter, held here by Wing Commander Alan Turton. This model was designed for extended operating times and featured a completely exposed cylinder head for cooling. Its all up weight was around 10 kilograms. (Photo taken in 1976). the shrouding effect of the larger cowling and to allow sufficient air mass flow to provide some thrust from the prop. Thus the prop will blow out to say a 12x6 or even a 13x5. This will immediately drop the revs and raise the operating head temperature. Worse still, the muffler may be enclosed inside the cowling for the sake of appearance The muffler will also more than likely be a simple box type with no airflow passing through for cooling or extraction of the hot exhaust gases. Even worse, if the modeller tries to use scale exhausting and channels the exhaust gases out via the scale exhaust openings, there is a distinct risk of the motor re-breathing burnt, hot exhaust gases instead of fuel-rich, cool air. Here we have a potentially ·lethal combination. A slower flying aircraft, a motor swinging a larger load, hotter exhaust gases, possible ingestion of hot exhaust gases into the carburettor and very little cooling air over the cylinder head and muffler all combining to raise the head temperature to beyond the safe limits. The result can be a seized motor and a very expensive repair. Here the need for maximum oil is of paramount importance. Different types of motors also have their own lubrication needs. Motors with piston rings have vastly different characteristics to the lapped piston/cylinder or even the ABC (aluminium/brass/chrome) piston/cylinder liners. There are other factors which confuse the issue even further. Often you will hear a modeller say, "but I have been using the same motor, same fuel and model for years, so why has the motor seized now?" The answer is that the airflow patterns inside a cowling are very complex and even the type of manoeuvres being practised on any one day can alter the flow patterns enough to raise the temperature inside the cowling. Add to this a continual build up of cylinder and cooling fin glaze from old burnt oil, a hotter than usual day and a needle setting that's just a little leaner than usual, and bang goes an engine. The problem of successfully cowling scale models is one that has plagued the modelling fraternity ever since scale modelling began. The basic problem in the above scenario is of course the wrong choice of motor to begin with. A Schnurle ported motor should not be used in this situation. A 4-stroke motor which will swing a much larger prop at much lower cylinder head temperatures would be a bE)tter choice. But model- These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A 11 .95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l ----------Enclosed is my cheque/money order for $_ _ _ or please debit my O Bankcard 0 Visa O Mastercard I Card No: Card Expiry Date __/__ Signature _ _ _ _ _ _ _ _ __ I Name _ _ __ __ _ _ _ __ I Address_ _ _ _ _ _ _ _ __ . __________ _ - - - - - . , . . - - P/code_ __ Q CTOllE/l 1992 89 Engines for remote controlled models - ctd lers will insist on using what they have at hand or they just simply cannot afford to buy a new motor. Whatever the reasons, the above scenario arises time after time on model flying fields. Model boats The problem of engine overheating in model boats is virtually non-existent for a more than adequate supply of cooling water is always at hand. In fact, the reverse is often the case, where too much water is pumped through the water scoop and the motor is over-cooled to the detriment of performance. Some care must be exercised in starting before the model is placed in the water but, by and large, the minimum oil ratios may be used in a boat. On the other hand, model cars present a formidable challenge as an adequate supply of air or water is seldom· available. In this case, oversize heatsinks are used on the motor and the maximum airflow available is piped inside the car body. Oil ratios are of the utmost importance in these models. Some land models, such as tanks, use closed circuit water cool- battery, is reliable and is quite powerful. It still has its adherents in control line and free flight. The fuel is typically 30% ether, 30% kerosene and 40% castor oil. It can be a little tricky to start for beginners as there are two running controls to adjust, both of which interact: (1) the regular needle valve, used to adjust the fuel/air mixture; and (2) the sub piston or contra piston, which is adjusted by the compression screw on the top of the cylinder. For a twiddler, this situation is dynamite and I have seen them land themselves in all sorts of bother because they are constantly twisting the compression screw and the needle valve. The basic rule with diesels is once the compression screw is set at the running position, never change it more than one half of one turn in the reduced compression direction and never adjust the needle valve. If weather changes or some out of the ordinary change forces an adjustment, never move the needle more than one turn. If the correct settings are lost, they can be difficult to find. This also applies to the first start of a new engine. "A good bright spark cures many ills in a model engine and these motors perform reliably & are quite long lasting" ing systems complete with radiator, but they are rare. Thus the answer to the question, "how much oil?", is just simply another question, "in what application?". Types of motor Turning now to the common types of motors in use in models, let's begin with the diesel. This type of motor has largely fallen by the wayside in the field of R/C modelling. This is largely due to the fact that it is difficult to provide a throttle control and if a throttle is fitted, the idle is not reliable. Model diesels are almost invariably 2-stroke. The diesel is cheap to run, does not require a starting 90 SILICON CHIP Once set up correctly, however, the diesel can be a most reliable starter and the ·old Mills 0. 75cc diesel is the most reliable motor I have ever had the pleasure of flying with. First flick starts are almost guaranteed and for this reason the Mills 0. 75 is still first choice for the power scramble, a freeflight event in which the winner is the flyer logging the most minutes of flight time in a 1-hour period, most of which can be spent scrambling after the model. This event is great fun for kids and mature age kids as well. It's a pity more R/C flyers do not indulge in this event. It certainly deflates much of the pomposity found on some R/C flying fields. It brings us back to the original concept of an outdoor activity in a country setting, providing physical and mental exercise, companionship, low cost and straight out fun! The only event to top it is the night scramble. The sight of 10 or 20 little fireflies buzzing around under a full moon, pursued by sometimes inebriated and usually overweight torch bearers, can be quite entertaining. The continual drift of conversation from out of the darkness, expletives and the odd splash, followed by "who put that #<at>&! river there?", all added to the sense of mischief and drama. Those were the good old days! Spark ignition motors The spark ignition motor is for enthusiasts and has a good following amongst those who know exactly what they want from a motor. As stated last month, a good spark cures many ills in a model engine and these motors perform reliably and cheaply, and are long lasting. The fuel is usually petrol/mineral oil but methanol/castor oil may also be used. Efficiency is better on petrol and fuel mixes of 40:1 (40 parts of petrol to 1 part of oil) are quite common. Methanol fuels for spark ignition usually run around 6:1 to 10:1 (10 parts methanol to one part castor oil). As a general guide, the smaller the engine the higher the oil content. These motors find their way into a lot of large models and capacities of up to 160cc are quite common. Modified chain saw motors provide a very cheap source for these large motors. The low cost of the fuel, combined with the very low oil residues on the model, are two of the favourite · virtues quoted by the enthusiast. Spark ignition motors throttle well but the throttle arrangement is a little more cumbersome, as timing retardation must be provided in addition to the normal carburettor control. Modern spark ignition motors are routinely run with electronic ignition and the timing can be provided from a variety of sensors - Hall Effect and light beam sensors being the two most popular. The throttle control moves the position of the pick-up head in relation to the timing disk. Because model engines run at very high revs, magnetic pick-ups are not particularly successful. The sensor is usually mounted on the prop driver, close to the hub of the prop or fly- wheel. The electronic ignitions usually run on 4.5-6V and most modellers seem to use the same coils that were fitted to the original spark interrupter systems. As stated last month, great care must be exercised when using petrol-based fuels. Spark ignition may be applied to 2stroke or 4-stroke motors. However, there is a major advantage with petrol fuels in 4-strokes because the very small clearances used in these motors can be badly fouled by burnt castor oil, particularly if the motor has not been run for some time. Burnt castor oil ages into a rubbery compound which can melt under heat into a very hard glaze. This is difficult to remove and, for this reason, after-run cleaning is important with 4-stroke motors. The low content of mineral oil in petrol-based fuels means that any residue is much easier to remove and does not age as badly. But there is one saving grace for castor oil and that is, it is still one of th~ best upper cylinder (high temperature) lubricants available and you delete castor oil from your fuel at your own peril. The typical fuel mix for methanol based 4strokes is 10% nitromethane, 10-12% castor oil and the rest methanol. Glowplug motors By far the most popular motor for R/C work is the glowplug motor, either 2-stroke or 4-stroke. The 4-stroke has gained greatly in popularity over the past few years as reliability and performance have gradually improved. They are very popular with the scale fraternity due to the more realistic exhaust note and lower noise levels. The mufflers are also much smaller and thus easier to fit into scale models, while the lower head temperatures are a boon for operation inside fully enclosed cowlings. They also swing larger props at lower revs, thereby further reducing noise levels due to the lower tip speed of the propeller. They do, however, suffer from several drawbacks. These drawbacks include higher cost, greater complexity, increased maintenance and lower output power when compared to the modern highly-developed 2-stroke engine. However, to those modellers who know exactly what they want from a model, the foregoing inconveniences are easily outweighed by One in a Thousand... speaker that performs with equal excellence on both classical and A rock music is a rare speaker indeed. 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Black or mahogany R.R.P...... ....... ... .... ......... .. $2500 per pair For a free information kit on the full range of DALi speakers, please contact Australian Distributor: SCAN AUDIO, 52 Crown Str, Richmond VIC 3121. the benefits obtained and there are many fine models now powered by these little engineering masterpieces. The workhorse of the model movement is still the 2-stroke glowplug engine, developed these days to give staggering performance. Light in weight, simple in construction and . easy to start, they are extremely powerful, very easy to throttle and give reliable operation at low revs. With all that, the modern glowplug motor is a far cry from the old Frog 160, my first glowplug motor purchased in the early 1950s. Ph: (03)429 2199. Fax (03)429 9309 I only ever got that motor running once and then I got such a shock after weeks of futile flicking that I forgot to remove my thumb from the propeller disc and it went through that thumb like a bacon slicer. It was a lesson I never forgot and I have treated all motors with great respect from that point on. Keep in mind that the Frog 160 was 1.6cc in capacity, a tiny motor by modern standards. Motors of 160cc are commonplace these days and are capable of inflicting serious injury. They must always be treated with great respect. SC Ocrn!l1-:n 1992 91